On shrinking up the window, ‘Open new tab’ button and ‘Switching between open tabs’ (I actually didn’t knew what exactly it’s called. It’s the arrow button used to switch open tabs in the line, when they can’t fit the title bar) button collapses.
this category for learning about web technology
check this link for known bug or report them
as it would be better to give more details about what you facing like you browser version and platform (windows / linux/android or what) and your platform version and how to reproduce this issue video or screen shoot would help
but before that try to create new profile open new tab and type about:profiles then create new profile and see if you have same issue with that new profile or not
also sometime clear cache and history help and also disable all extension